x86/svm: ignore accesses to EX_CFG

x86/svm: ignore accesses to EX_CFG

Windows 10 will try to unconditionally read (and possibly even adjust)
EX_CFG on AMD hardware, despite it being documented only for Fam15 models
0xh, and injecting a #GP fault will result in a panic:

svm.c:1964:d5v0 RDMSR 0xc001102c unimplemented
d5v0 VIRIDIAN CRASH: 7e ffffffffc0000096 fffff8054cbe5ffe fffffa0837a066e8 fffffa0837a05f30

Return 0 when trying to read the MSR and drop writes.

Fixes: 84e848fd7a16 ('x86/hvm: disallow access to unknown MSRs')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>